296 SOLID PARTS OF ANIMALS* 



sions the extractive bears a greater proportion to the tannin. 

 Davy has ascertained, that the hides extract both the tannin and 

 extractive from the infusion, and leave nothing behind but pure 

 water, provided they be employed in sufficient quantity. Hence, 

 it is obvious, that both the tannin and extractive must enter into 

 the composition of leather. The extractive gives the hide a brown 

 colour, but does not render it insoluble in boiling water ; the tan- 

 nin renders it insoluble, but its colour continues whitish. Hence 

 it is likely that the lightest kinds of leather contain little else than 

 tannin, while the brown kinds contain both tannin and extractive, 

 and the new compound is leather. Hence the reason of the in- 

 crease of its weight. 



Davy found that 100 of calf skin absorbed 64 in weight from 

 a concentrated infusion of nut-galls, and 34 from a concentrated 

 solution of oak bark ; 1 7 in a dilute solution of the same bark ; 

 34 in concentrated, and 15 in a dilute infusion of catechu. It 

 is generally admitted that 100 parts of skin, when tanned, be- 

 come 140 parts in weight. 



Calf-skins, and those hides which are prepared by the grainer, 

 are first steeped in weak infusions of oak bark, and gradually re- 

 moved to stronger and stronger, till they are completely impreg- 

 nated, which takes up from two to four months. As the weak 

 infusions contain a greater proportion of extractive, the conse- 

 quence of this process is, that the skin combines in the first place 

 with a portion of it, and afterwards with the tannin. When sa- 

 turated solutions of tannin are employed, the leather is formed 

 in a much shorter time. This was the process recommended by 

 Seguin ; but it has been observed, that leather tanned in this way 

 is more rigid and more liable to crack than leather tanned in the 

 usual way. Hence it is likely, as Davy has observed, that the 

 union of the extractive is requisite to form pliable and tough lea- 

 ther. Leather rapidly tanned must be less equable in its texture 

 than leather slowly tanned, as the surface must be saturated with 

 tannin before the liquid has time to penetrate deep. Davy has 

 ascertained that skins, while tanning, seldom absorb more than 

 one-third of their weight of vegetable matter. 



Skins intended for sole leather are generally kept from the 

 first in an infusion preserved nearly saturated by means of the 

 strata of bark with which they alternate. The full impregnation 

 requires from ten to eighteen months. It is likely, from this 



